1. Field of the Invention
The present invention relates to the lithographic exposure and patterning of workpieces, and in particular relates to an ink-based method and apparatus for masking a workpiece to prevent exposure of select regions on the workpiece.
2. Description of the Prior Art
Lithography techniques are used in the manufacturing of microdevices, such as, integrated circuits (ICs), flat panel displays, microelectromechanical systems (MEMS), the formation of bump IC interconnects for “flip chip” interconnection technology, and the like. The lithographic process involves the use of photosensitive workpieces (“wafers”) and the selective exposure of such workpieces with radiation (e.g., UV light). Workpiece photosensitivity is typically achieved by coating or otherwise applying a layer of photosensitive material called photoresist to the workpiece surface. Photoresist can either be “positive-tone,” in which the exposed resist is removed upon developing, or “negative-tone,” in which the unexposed resist is removed upon developing. Generally, the lithography process includes the steps of coating the workpiece with photoresist, exposing the photoresist with the image of a mask to form a latent pattern in the photoresist, developing the photoresist to form a three dimensional image, etching the photoresist to form a corresponding three dimensional pattern in the workpiece, and then removing the excess photoresist. These steps are repeated a requisite number of times to form the particular device structure in the workpiece.
In certain lithography applications involving negative photoresist, it is preferred that select regions of the workpiece outside of the individual exposure fields remain completely unexposed so that the resist in these regions is removed upon developing. One example application is bump interconnect lithography, which involves the formation of conductive (e.g., gold or aluminum) bumps on a workpiece (wafer) that are used to contact circuit lines on a circuit board. Bump lithography includes an electrochemical plating step to form the conductive bumps that requires contacting most of the edge of the wafer with an electrode. For this purpose, substantially all of the edge of the wafer must be free of all photoresist to ensure uniform electrical contact.
When one-to-one contact or near-contact photolithography is used the entire wafer is exposed at once and the exclusion of the edge is achieved by incorporating the desired exclusion into the mask pattern. However, in step and repeat photolithography it is not possible to define the exclusion area into the mask pattern since the stepping pattern depends on the pattern size and the step size.